Atrioventricular block (first, second, or third degree)
- Increased vagal tone (first degree or Mobitz type I)
- Drugs: digitalis, calcium channel blockers, beta blockers, clonidine, other sympatholytics, amiodarone, other antiarrhythmics
- Age-related conduction system fibrosis or calcification (Lev's disease)
- Cardiac infiltrative disease (e.g., amyloid, sarcoidosis, hemochromatosis)
- Myocardial ischemia or infarction
- Chagas' disease
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